The present disclosure relates to a storage element with a configuration that has perpendicular magnetic anisotropy with respect to the film plane and obtains magnetoresistance change through perpendicular current application, a method for manufacturing a storage element, and a memory including this storage element.
Along with dramatic development of various kinds of information apparatus ranging from high-capacity servers to mobile terminals, further enhancement in the performance, such as increases in the degree of integration and the speed and power consumption reduction, is pursued also regarding elements such as memory and logic that configure the information apparatus. In particular, the advance of the semiconductor non-volatile memory is significant and the spread of the flash memory as a large-size file memory is progressing at such a rapid pace as to drive out the hard disk drive. On the other hand, in anticipation of expansion into the code storage and the working memory, development of ferroelectric random access memory (FeRAM), magnetic random access memory (MRAM), phase-change random access memory (PCRAM), etc. is being advanced to replace NOR flash memory, DRAM, etc., which are generally used presently. Part of these memories has been already put into practical use.
In particular, the MRAM is capable of high-speed and almost-infinite (1015 times or more) rewriting because data is stored based on the magnetization direction of a magnetic material. The MRAM has been already used in the fields of the industrial automation, the airplane, etc. Because of its high-speed operation and high reliability, the MRAM is expected to be expanded into the code storage and the working memory in the future. However, it still has challenges in power consumption reduction and capacity increase in practice. They are essential challenges attributed to the principle of storage in the MRAM, i.e. the system in which magnetization reversal is caused by a current magnetic field generated from an interconnect.
As one method to solve this problem, studies are being made on a system of storage, i.e. magnetization reversal, that is not based on the current magnetic field. Particularly, researches relating to spin torque magnetization reversal are active. A storage element by the spin torque magnetization reversal is configured with magnetic tunnel junction (MTJ) as with the MRAM. This configuration utilizes a characteristic that spin-polarized electrons passing through a magnetic layer pinned to a certain direction give torque to another magnetic layer that is free (whose direction is not fixed) when entering this magnetic layer. In this configuration, application of a current equal to or larger than a certain threshold value causes reversal in the free magnetic layer. Rewriting of 0/1 is performed by changing the polarity of the current. The absolute value of the current for this reversal is equal to or smaller than 1 mA in an element with scale of about 0.1 μm. In addition, scaling is possible because this current value decreases in proportion to the element volume. Moreover, this system does not require the word line for generating the current magnetic field for storage, which is necessary for the MRAM, and therefore also has an advantage that the cell structure may be simpler.
Hereinafter, the MRAM utilizing the spin torque magnetization reversal will be referred to as the ST-MRAM (spin torque-magnetic random access memory). Great expectations are placed on the ST-MRAM as a non-volatile memory that enables power consumption reduction and capacity increase while keeping the MRAM's advantages that high-speed operation is possible and the number of times of rewriting is almost infinite.
To achieve power consumption reduction in the ST-MRAM, it will be important to lower the resistance of the MTJ element by decreasing the thickness of the tunnel barrier layer. If the size of the element is reduced for capacity increase in accordance with scaling, the resistance due to the tunnel barrier layer increases. Thus, decreasing the resistance of the element is difficult. Therefore, the thickness of the tunnel barrier layer should be decreased in order to achieve both of scaling and power consumption reduction.
FIG. 11 is a schematic sectional view of the MTJ structure of an ST-MRAM. The ST-MRAM shown in FIG. 11 is composed of a base layer 51, an antiferromagnetic layer 52, a magnetization pinned layer (pinned layer, reference layer) 53, a tunnel barrier layer 54, a storage layer (magnetization storage layer, free layer) 55, and a cap layer (protective layer) 56. The arrowheads indicate the magnetization directions of the respective magnetic layers in ST-MRAM operation. The storage layer 55 is formed of a ferromagnetic material having a magnetic moment whose direction of magnetization M55 freely changes to a direction parallel to the film plane (horizontal direction). The magnetization pinned layer 53 is formed of a ferromagnetic material having a magnetic moment whose direction of magnetization M53 is a fixed direction parallel to the film plane (horizontal direction). Information is stored based on the magnetization direction of the free magnetization layer (storage layer) having uniaxial anisotropy. Writing is performed by applying a current in a direction perpendicular to the film plane to thereby cause spin torque magnetization reversal in the storage layer.
The MTJ element has an extremely-thin tunnel barrier layer with a thickness equal to or smaller than 1 nm. Therefore, the interface of the tunnel barrier layer is sensitive to roughness and should be sufficiently flat. As a technique to planarize an MgO layer generally used as the tunnel barrier layer, e.g. a method of performing heating in a vacuum after forming an MgO film is known (refer to e.g. Isogami et al., APPLIED PHYSICS LETTERS Vol. 93, 192109 [2008] as Non-Patent Document 1).